1. Field of the Invention
This invention relates to a laser wavelength controlling circuit for controlling the wavelength of laser light, and more particularly to a wavelength controlling circuit for a laser signal having a rough adjustment function of an oscillation wavelength of laser light.
2. Description of the Relates Art
Investigations of a wavelength multiplex method as an optical transmission system are performed energetically at present, and a demand for a technique for transmitting many wavelength signals is increasing. However, the wavelength band of a signal which can be used by an optical amplifier is restricted, and in order to allow the restricted wavelength band to have a number of channels as great as possible, it is necessary to narrow the wavelength interval and stabilize the oscillation frequency of a laser signal with a high degree of accuracy.
Conventionally, as a method of stabilizing the oscillation frequency of a laser signal of an optical transmitter, a method of controlling the temperature of a laser module is used. For example, according to Japanese Patent Laid-Open No. 302948/1995, a wavelength stabilization apparatus is disclosed wherein, as shown in FIG. 1, temperature sensor 20 and Peltier element 22 are disposed in the proximity of semiconductor laser 14, and temperature controller 26 which drives Peltier element 22 in response to an output of temperature sensor 20 is provided. Further, polarizing beam splitter 28, xc2xc wavelength plate 32, etalon 34 and photodiode 36 are disposed in order in front of light source unit 12, and current controller 38 which supplies inrush current to semiconductor laser 14 in response to an output from photodiode 36 is provided to stabilize the wavelength of light to be emitted.
Meanwhile, according to Japanese Patent Laid-Open No. 37334/1996, a laser wavelength controlling apparatus is disclosed wherein, as shown in FIG. 2, laser diode 42 and Peltier effect element 41 which controls the temperature of laser diode 42 are disposed in LD module 43. Laser light 45 generated from LD module 43 is introduced into optical narrow band high-pass filter 46 and optical narrow band low-pass filter 51, and transmission light through them is detected by photodiodes 48, 53, respectively. Filters 46, 51 transmit light components at wavelength positions displaced a little by an equal wavelength to the longer wavelength side and the shorter wavelength side from a desired oscillation center wavelength of the laser light. Accordingly, if Peltier effect element 41 is controlled by temperature adjustment circuit 58 so that the light components detected by photodiodes 48, 53 may be equal to each other, the laser light can be set to the desired oscillation center frequency.
However, the former wavelength stabilization apparatus has a problem in that, even if the temperature is controlled to a fixed value, the wavelength suffers from a drift by secular degradation of the semiconductor laser or the temperature sensor.
Meanwhile, with the latter laser wavelength controlling apparatus, although it is improved against a drift of the wavelength by secular degradation of the laser diode or the temperature sensor, it is not easy to set it to an intended wavelength.
It is an object of the present invention to provide a wavelength controlling circuit for a laser signal which can set a laser signal to a target wavelength readily upon wavelength control of the laser signal and eliminates the problem of a drift.
According to an aspect of the present invention, a wavelength controlling circuit for a laser signal comprises a laser diode module including a semiconductor laser diode, a temperature detection element and a temperature adjustment element, wavelength controlling means including an optical band-pass filter used as a wavelength discrimination element for controlling the wavelength of a laser signal within a range of fine adjustment within which the wavelength can be discriminated by the optical band-pass filter, and temperature controlling means for adjusting, when the wavelength of the laser signal is outside the range within which the wavelength can be discriminated with a wavelength discrimination characteristic by the optical band-pass filter, the wavelength of the laser signal roughly so that the wavelength may be included in the discrimination allowing range.
According to another aspect of the present invention, a wavelength controlling circuit for a laser signal comprises a laser diode including a semiconductor laser diode, a temperature detection element and a temperature adjustment element, monitoring means including an element which branches part of a laser signal output of the semiconductor laser diode and a wavelength discrimination element for monitoring the wavelength of the laser signal, first adjustment means for generating a signal for finely adjusting the wavelength of the laser signal in response to an output from the monitoring means, second adjustment means for generating a signal for roughly adjusting the wavelength of the laser signal in response to a result of monitoring of the temperature which is monitored by the temperature detection element of the laser diode module, switching means for discriminating, in response to the result of the temperature monitoring of the laser diode module, that a predetermined wavelength is reached and performing switching from the second adjustment means to the first adjustment means, and driving means for driving the temperature adjustment element of the laser diode module with the adjustment signals of the first and second adjustment means.
According to a further aspect of the present invention, a wavelength controlling circuit for a laser signal comprises a laser diode including a semiconductor laser diode, a temperature detection element and a temperature adjustment element, an element for branching part of a laser signal output from the semiconductor laser diode, a wavelength monitoring circuit including a beam splitter for splitting the branched laser signal into two laser signals, an optical band-pass filter having a different transmission factor of an optical wavelength for receiving one of the split laser signals, a first optical detector for receiving the laser signal having transmitted through the optical band-pass filter, and a second optical detector for receiving the other of the split laser signals, the wavelength monitoring circuit monitoring a range within which the wavelength of the laser signal can be discriminated by the optical band-pass filter, a first controlling circuit for generating a signal for finely adjusting the wavelength of the laser signal, the first controlling circuit including a divider for dividing an output from the second optical detector of the wavelength monitoring circuit by an output from the first optical detector and a first amplifier for amplifying a difference voltage between an output signal from the divider and a first reference voltage, a second controlling circuit for generating a signal for roughly adjusting the wavelength of the laser signal so that the wavelength of the laser signal may come within a discrimination allowing range, the second controlling circuit including a temperature monitoring circuit connected to a thermistor of the laser diode module for detecting a voltage signal proportional to the temperature after operation is started and a second amplifier for amplifying a difference between an output from the temperature monitoring circuit and a second reference voltage, a switching circuit for performing switching from the second controlling circuit which roughly adjusts the wavelength of the laser signal to the first controlling circuit which finely adjusts the wavelength of the laser signal, the switching circuit including a comparison circuit for receiving the voltage proportional to the temperature by the rough adjustment of the wavelength of the laser signal and generating an activation signal with a predetermined voltage and a switch which operates in response to the activation signal of the comparison circuit, and a driving circuit for driving the temperature adjustment element of the laser diode module with the fine adjustment signal of the first controlling circuit.
With the present invention, there is an effect that a wavelength drift by secular degradation of a semiconductor laser or secular change of a thermistor is eliminated. The reason is that, as a method of directly detecting and feeding back the wavelength of a laser signal, a wavelength monitor module which makes use of a transmission characteristic of an optical filter is used for control.
Further, with the present invention, whatever the ambient temperature or the like upon starting of control is, the wavelength can be initially set to a value within a control allowing range and wavelength control of a high degree of reliability can be achieved without a malfunction. The reason is that the two controlling circuits of a wavelength controlling circuit and a temperature controlling circuit wherein an optical band-pass filter is used as a wavelength discrimination element are included.
Accordingly, from the reasons described above, the present invention has an effect that the full width at half minimum of the optical band-pass filter can be narrowed and wavelength control of a high degree of accuracy can be achieved.